1. Technical Field
The present invention relates to a telecommunications system, in particular to a telecommunications system which includes a mobile network.
It is known for portable communication devices to attach to different points on a network. Thus, for example, a user can connect a portable computer to one attachment point, move to a new geographical location where there is provided another attachment point and attach the computer to the network at that new attachment point. Where users travel together, for example on a common vehicle such as a train or aeroplane, it can be advantageous for the communication-equipment of each user to be connected to, a local network, such as a Local Area Network (LAN) within the vehicle. As well as being able to communicate amongst each other, the users will normally be able to receive common services from the local network. Furthermore, if the attachment to the main network is through a radio link, and the local network includes a transmitter/receiver, the communication devices may take advantage of the transmitter/receiver provided by the local network.
2. Related Art
So that the communication devices can be reached at different attachment points on the main network, a directory facility such as a “Home Agent” can be provided in the main network. The directory facility for each communication device normally stores information which can be used to facilitate the routing of data to the current attachment point of the communication device. The directory facility may contain the address corresponding to the current attachment point, or the directory facility may contain another address indicative of a location where further routing information can be found. When the attachment point of a communication device changes, the communication device will normally send a message to the directory facility associated with it, so that the routing information stored in the directory facility can be updated to reflect the change in the attachment point of the communication device.
However, in the case where a plurality of communication devices move together as part of a local or mobile network, the amount of signalling required between the communication device's and their respective directory facilities when the point of attachment changes can be undesirable.